The present invention relates to a novel powder coating composition capable of forming a multilayer film excellent in finished appearance, weather resistance and other properties.
Powder coating compositions do not necessitate organic solvents and thus are advantageous from the viewpoints of environmental protection and saving of resources. They are therefore widely utilized in industrial products such as electric appliances, automobiles, vehicles, office goods, steel furniture and construction materials.
Japanese Examined Patent Publication No. 14577/1978 and Japanese Unexamined Patent Publication No. 105135/1979 disclose powder coating compositions prepared by dry blending a lower layer-forming thermosetting powder coating material and an upper layer-forming thermosetting powder coating material. The powder coating compositions are applied to a substrate by powder coating and then thermally melted to form a multilayer film.
In the above powder coating compositions, mainly used as the upper layer-forming coating material are acrylic resin thermosetting powder coating materials or polyester resin thermosetting powder coating materials which are resistant to deterioration by sunlight and excellent in film appearance and weather resistance, but poor in corrosion resistance. Chiefly used as the lower layer-forming coating material are epoxy resin-based thermosetting powder coating materials which are excellent in corrosion resistance and adhesion to the substrate but poor in weather resistance.
However, these conventional powder coating compositions for forming a multilayer film have the drawback that, when the compositions are applied to the substrate by powder coating and heated to form a multilayer film, they do not sufficiently separate into upper and lower layers and thus result in a multilayer film poor in finished appearance, weather resistance and other properties.
For removing such a drawback, Japanese Examined Patent Publication No. 21545/1988 proposes a method for forming a multilayer film comprising the steps of surface-treating a substrate with an onium salt compound and applying a powder coating composition for forming a multilayer film prepared by dry blending a lower layer-forming thermosetting powder coating material and an upper layer-forming thermoplastic powder coating material. The proposed method, however, is industrially disadvantageous because the surface treatment increases the number of steps involved in the method. Further, the method requires troublesome procedures to produce suitable conditions for the surface treatment.
An object of the present invention is to provide a novel powder coating composition for forming a multilayer film, which is free from the above drawbacks of the prior art.
Another object of the present invention is to provide a novel powder coating composition capable of forming a multilayer film excellent in finished appearance, weather resistance and other properties.
Other objects and features of the present invention will be apparent from the following description.
The present invention provides a powder coating composition for forming a multilayer film, obtainable by dry blending a lower layer-forming thermosetting powder coating material (A) and an upper layer-forming thermosetting powder coating material (B), wherein at least one of the following conditions is satisfied: (I) the material (A) contains an onium salt compound; (II) particles of 45 xcexcm or smaller diameter account for at least 90 wt. % of each of the materials (A) and (B); and (III) the material (A) is higher than the material (B) in melt viscosity (Paxc2x7s) measured at 130xc2x0 C.
The present inventors conducted extensive research to solve the above problems of the prior art and found that a powder coating composition satisfying one of the above conditions (I), (II) and (III) necessitates no surface treatment and is capable of forming a multilayer film excellent in finished appearance, weather resistance and other properties, since the composition sufficiently separates into upper and lower layers.
The present invention has been accomplished based on these novel findings.
It is a matter of course that a powder coating composition satisfying two or all of the conditions (I), (II) and (III) achieves equivalent or superior results.
The powder coating composition satisfying the condition (I) is a powder coating composition obtainable by dry blending a lower layer-forming thermosetting powder coating material (A) and an upper layer-forming thermosetting powder coating material (B), wherein the material (A) contains an onium salt compound. Such a powder coating composition is capable of sufficiently separating into upper and lower layers and forming a multilayer film with excellent properties.
Usable onium salts include those represented by the formula:
[(R)4Y]+Xxe2x88x92xe2x80x83xe2x80x83(1)
or the formula:
[(R)3S]+Xxe2x88x92xe2x80x83xe2x80x83(2)
wherein R""s are the same or different and each represent hydrogen, lower alkyl, hydroxy lower alkyl, halo lower alkyl, lower alkoxy lower alkyl, cycloalkyl, aryl, aralkyl or like organic group; Y represents a nitrogen atom or phosphorus atom; X represents an anionic ion such as a halogen ion, inorganic acid group, organic acid group or the like. In the above definitions, xe2x80x9clowerxe2x80x9d means a carbon number of 6 or less.
Examples of lower alkyl include methyl, ethyl, propyl, butyl and hexyl. Examples of hydroxy lower alkyl include hydroxymethyl, hydroxyethyl, hydroxypropyl, hydroxybutyl and hydroxyhexyl. Examples of halo lower alkyl include bromomethyl and bromoethyl. Examples of lower alkoxy lower alkyl include methoxymethyl, methoxyethyl, methoxypropyl, methoxybutyl and methoxyhexyl. Examples of cycloalkyl include cyclohexyl, cyclohexylmethyl and cyclopentyl. Examples of aryl include phenyl, toluyl and xylyl. Examples of aralkyl include benzyl. Examples of halogen ions include chlorine ion, bromine ion, fluorine ion and iodine ion. Examples of inorganic acid groups include sulfuric acid group and phosphoric acid group. Examples of organic acid groups include acetic acid group, benzylsulfonic acid group and hydroxyl group. In the above formulas, R is preferably lower alkyl, phenyl or benzyl, and X is preferably a halogen ion. Preferable examples of onium salt compounds include ammonium salt compounds and phosphonium salt compounds.
Specific examples of onium salt compounds include tetramethyl phosphonium chloride, tetraethyl phosphonium chloride, tetrabutyl phosphonium chloride, trimethylethyl phosphonium chloride, triphenylbenzyl phosphonium chloride, tetramethyl phosphonium bromide, triphenylbenzyl phosphonium bromide and like phosphonium salt compounds; tetramethyl ammonium chloride, tetraethyl ammonium chloride, tetrabutyl ammonium chloride, trimethylethyl ammonium chloride, triphenylbenzyl ammonium chloride, tetramethyl ammonium bromide, triphenylbenzyl ammonium bromide and like ammonium salt compounds; and trimethyl sulfonium chloride, tetraethyl sulfonium chloride, tetrabutyl sulfonium chloride, trimethylethyl sulfonium chloride, triphenylbenzyl sulfonium chloride and like sulfonium salt compounds.
The proportion of the onium salt compound is preferably 0.01 to 10 parts by weight, particularly 0.01 to 5 parts by weight, relative to 100 parts by weight of the base resin of the material (A). If the proportion is less than 0.01 parts by weight, the resulting composition does not sufficiently separate into upper and lower layers and the boundary of the two layers become uneven, reducing smoothness and gloss of the film surface. On the other hand, a proportion exceeding 10 parts by weight does not further improve the separability of the composition, hence undesirable. When the material (A) is a thermosetting epoxy resin powder coating material, it is preferable to use the onium salt compound in a proportion of 0.01 to 2.0. parts by weight, particularly 0.01 to 1.0 parts by weight, relative to 100 parts by weight of the base resin, since the onium salt compound acts as a curing catalyst for the powder coating material. In this case, proportions outside the range from 0.01 to 2.0 are undesirable since the resulting coating composition has reduced separativity and forms a film poor in appearance characteristics such as smoothness and gloss and in performance characteristics such as weather resistance and curability.
The onium salt compound can be incorporated into the material (A) by melt blending or dry blending, preferably by melt blending.
The powder coating composition satisfying the condition (II) is a powder coating composition obtainable by dry blending a lower layer-forming thermosetting powder coating material (A) and an upper layer-forming thermosetting powder coating material (B), wherein particles of 45 xcexcm or smaller diameter account for at least 90 wt. % of each of the materials (A) and (B). Such a powder coating composition is capable of sufficiently separating into upper and lower layers and forming a multilayer film with excellent properties. If the proportion of particles of 45 xcexcm or smaller diameter is less than 90 wt. % and particles of a diameter larger than 45 xcexcm are present in a relatively large amount, the resulting composition has reduced separability, so that when the composition is thermally melted to form a multilayer film, part of the material (A) will rise to the surface of the upper layer formed from the material (B) and impair the appearance and performance characteristics of the film.
It is preferable that particles of 5 xcexcm to 45 xcexcm diameter, more preferably 10 xcexcm to 40 xcexcm diameter, account for at least 90 wt. %, particularly at least 95 wt. %, of each of the materials (A) and (B). If particles of 5 xcexcm or smaller diameter are present in a relatively large amount, the application workability tends to reduce owing to electrostatic repulsion or other factors.
To obtain the materials (A) and (B) wherein particles of 45 xcexcm or smaller diameter account for at least 90 wt. %, classification by sieving can be employed, for instance.
The powder coating composition satisfying the condition (III) is a powder coating composition obtainable by dry blending a lower layer-forming thermosetting powder coating material (A) and an upper layer-forming thermosetting powder coating material (B), wherein the material (A) is higher than the material (B) in melt viscosity (Paxc2x7s) measured at 130xc2x0 C. Such a powder coating composition is capable of sufficiently separating into upper and lower layers and forming a multilayer film with excellent properties. If the material (A) is equal to or lower than the material (B) in melt viscosity, the material (A) will rise to the surface of the multilayer film, and causes film defects such as blurring or adversely affects performance characteristics of the film such as weather resistance and corrosion resistance. Some combinations of the materials (A) and (B) may produce films which appear to comprise separate layers when observed with the naked eye, even if the material (A) is equal to or-lower than the material (B) in melt viscosity. However, microscopic observation will reveal the presence of the material (A) on the surface of such films.
For separating the composition more completely into layers, it is desirable that the material (A) has a melt viscosity (Paxc2x7s) at least three times higher than that of the material (B). Further, it is suitable that the material (A) has a melt viscosity of 1 to 100 Paxc2x7s, preferably 5 to 70 Paxc2x7s, and the material (B) has a melt viscosity of 0.1 to 10 Paxc2x7s, preferably 1 to 8 Paxc2x7s.
As used herein, the melt viscosity (Paxc2x7s) is the value measured at 130xc2x0 C. using xe2x80x9cQuartz Reometer QRT-3000xe2x80x9d (tradename, a product of Tokyo Denpa Kiki K.K.).
The reason for measuring the melt viscosity of the materials at 130xc2x0 C. is that when, for example, a thermosetting acrylic resin powder coating material and thermosetting epoxy resin powder coating material in a dry blended composition are applied to a substrate and baked at 200xc2x0 C. (a temperature of the atmosphere in a heating furnace), they separate into layers usually at about 100 to 140xc2x0 C., although depending on the rate of temperature increase of the substrate.
The material (A) has a melt viscosity higher than that of the material (B) when, for example, the material (A) is higher in pigment concentration than the material (B), or when the base resin of the material (A) is higher in molecular weight than that of the material (B).
The thermosetting powder coating materials (A) and (B) may be any of known powder coating materials which are incompatible or poorly compatible with each other and capable of forming a multilayer film when thermally melted.
The lower layer-forming thermosetting powder coating material (A) used in the present invention may be a per se known powder coating material for forming a lower layer of a multilayer film. A thermosetting epoxy resin powder coating material is preferred as the material (A), since it is excellent in separability, corrosion resistance and adhesion to the substrate. Thermosetting epoxy resin powder coating materials will be specifically described below.
Thermosetting epoxy resin powder coating materials are coating materials which can be applied by powder coating and thermally cured, and which comprise an epoxy resin as a base resin and a curing agent for the epoxy resin.
The base resin may be, for example, a bisphenol-epichlorohydrin epoxy resin (e.g., xe2x80x9cEPIKOTE 1004xe2x80x9d and xe2x80x9cEPIKOTE 1007xe2x80x9d manufactured by Yuka Shell K.K.), a novolac epoxy resin, or the like. The resin has an epoxy equivalent of usually about 120 to 8000. Usable curing agents include, for example, adipic acid, trimellitic acid, trimellitic anhydride and like polycarboxylic acid compounds; benzyl-4-hydroxyphenylmethyl sulfonium hexafluoroantimonate and like aromatic sulfonium salts which serve as cationic polymerization catalysts; dicyandiamide and like amide compounds; adipic acid dihydrazide and like carboxylic acid dihydrazide compounds; imidazoline compounds; imidazole compounds; phenolic resins; and polyester resins with a high acid value.
The proportion of the curing agent to the base resin is as follows: When the curing agent is a cationic polymerization catalyst, the catalyst is used in a proportion of usually about 0.01 to 10 parts by weight, preferably about 0.1 to 5 parts by weight, relative to 100 parts by weight of the base resin. When a curing agent other than cationic polymerization catalysts is used, the proportion of the curing agent is usually about 10 to 100 parts by weight, preferably about 15 to 80 parts by weight, relative to 100 parts by weight of the base resin.
The material (A) may contain, where necessary, an anticorrosive agent, color pigment, extender pigment, filler, curing catalyst, fluidity modifier, anti-cissing agent or like additive for coating compositions.
The upper layer-forming thermosetting powder coating material (B) used in the present invention may be a per se known powder coating material for forming an upper layer of a multilayer film. A themosetting acrylic resin powder coating material or themosetting polyester resin powder coating material is preferred as the material (B), since these coating materials are excellent in weather resistance and processability. These coating materials will be specifically described below.
Thermosetting acrylic resin powder coating materials are coating materials which can be applied by powder coating and thermally cured, and which comprise an acrylic resin as a base resin and a curing agent for the acrylic resin. Examples of such powder coating materials include a powder coating material (a) comprising an epoxy-containing acrylic resin as a base resin and a polycarboxylic acid crosslinking agent as a curing agent; and a powder coating material (b) comprising a hydroxyl-containing arylic resin as a base resin and a blocked polyisocyanate crosslinking agent as a curing agent.
Usable as the base resin of the material (a) are epoxy-containing acrylic resins obtained by radically copolymerizing an epoxy-containing radically polymerizable unsaturated monomer and a hard acrylic monomer with a glass transition temperature of 40xc2x0 C. or higher, and where necessary, a soft acrylic monomer with a glass transition temperature lower than 40xc2x0 C., a radically polymerizable unsaturated monomer containing a functional group other than epoxy groups, and another radically polymerizable unsaturated monomer. The resin has an epoxy equivalent of usually about 120 to 8000. Examples of epoxy-containing radically polymerizable unsaturated monomers include glycidyl (meth)acrylate and methylglycidyl (meth)acrylate. Examples of hard acrylic monomers with a glass transition temperature of 40xc2x0 C. or higher include methyl methacrylate, ethyl methacrylate, iso-butyl methacrylate, tert-butyl methacrylate and tert-butyl acrylate. Examples of soft acrylic monomers with a glass transition temperature less than 40xc2x0 C. include methyl acrylate, ethyl acrylate, n-butyl methacrylate, iso-butyl acrylate, 2-ethylhexyl (meth)acrylate and stearyl methacrylate. Examples of radically polymerizable unsaturated monomers containing a functional group other than epoxy groups include hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate. Examples of other radically polymerizable unsaturated monomers include styrene, vinyl toluene, xcex1-methylstyrene, (meth)acrylonitrile and (meth)acrylamide. Examples of polycarboxylic acid crosslinking agents include adipic acid, azelaic acid, dodecanedionic acid, adipic anhydride and trimellitic anhydride.
Usable as the base resin of the powder coating material (b) are hydroxyl-containing acrylic resins obtained by radically copolymerizing a hydroxyl-containing radically polymerizable unsaturated monomer and a hard acrylic monomer with a glass transition temperature of 40xc2x0 C. or higher, and where necessary, a soft acrylic monomer with a glass transition temperature less than 40xc2x0 C., a radically polymerizable unsaturated monomer containing a functional group other than hydroxyl groups and another radically polymerizable unsaturated monomer. The resin has a hydroxyl value of usually about 20 to 200 mg KOH/g. Examples of hydroxyl-containing radically polymerizable unsaturated monomers include hydroxyethyl (meth)acrylate and hydroxypropyl (meth)acrylate. Examples of radically polymerizable unsaturated monomers containing a functional group other than hydroxyl groups include glycidyl (meth)acrylate and methylglycidyl (meth)acrylate. Examples of hard acrylic monomers with a glass transition temperature of 40xc2x0 C. or higher, soft acrylic monomers with a glass transition temperature less than 40xc2x0 C. and other radically polymerizable unsaturated monomers are as given above. The blocked polyisocyanate crosslinking agent is, for example, an aliphatic or alicyclic polyisocyanate compound wherein the isocyanate groups are blocked with a phenol, lactam, alcohol, oxime or like blocking agent. Examples of aliphatic or alicyclic polyisocyanate compounds include hexamethylene diisocyanate, trimethylene diusocyanate, isophorone duisocyanate and hydrogenated xylylene diisocyanate.
In the materials (a) and (b), the proportion of the curing agent to the base resin is usually about 10 to 100 parts by weight of the curing agent relative to 100 parts by weight of the base resin.
The thermosetting polyester resin powder coating material for use as the material (B) in the present invention is, for example, a powder coating material (c) which can be applied by powder coating and thermally cured, and which comprises a hydroxyl-containing polyester resin as a base resin and a blocked polyisocyanate crosslinking agent as a curing agent.
The base resin of the material (c) is, for example, a hydroxyl-containing polyester resin obtained by reacting an aromatic or alicyclic dicarboxylic acid and a dihydric alcohol, and where necessary, a monocarboxylic acid, a tri- or higher carboxylic acid and a tri- or higher hydric alcohol. The resin has a hydroxyl value of usually about 20 to 300 mg KOH/g. Examples of aromatic or alicyclic dicarboxylic acids include phthalic acid, phthalic anhydride, isophthalic acid, terephthalic acid, dimethyl isophthalate, dimethyl terephthalate, hexahydrophthalic acid, hexahydrophthalic anhydride, tetrahydrophthalic acid and tetrahydrophthalic anhydride. Examples of dihydric alcohols include (poly)ethylene glycol, (poly)propylene glycol, butylene glycol, neopentyl glycol, 1,6-hexanediol and dimethyl propionic acid. Examples of monocarboxylic acids include benzoic acid. Examples of tri- or higher carboxylic acids include trimellitic acid and trimellitic anhydride. Examples of tri- or higher hydric alcohols include trimethylol ethane, trimethylol propane, glycerine and pentaerythritol. Examples of blocked polyisocyanate crosslinking agents are as given above.
In the material (c), the proportion of the curing agent to the base resin is usually about 10 to 100 parts by weight, preferably about 15 to 80 parts by weight, of the curing agent relative to 100 parts by weight of the base resin.
The upper layer-forming thermosetting powder coating material (B) may contain, where necessary, an antimicrobial agent. The antimicrobial agent may be, for example, an inorganic antimicrobial agent comprising an inorganic compound and silver ions supported on the inorganic compound, or an organic antimicrobial agent such as zinc pyrithione.
Known inorganic compounds having silver ions supported thereon can be used as an inorganic antimicrobial agent without limitations. Examples of inorganic compounds for supporting silver ions include activated carbon, activated alumina, silica gel and other inorganic adsorbents, zeolite, hydroxy apatite, zirconium phosphate, titanium phosphate, potassium titanate, hydrated bismuth oxide and hydrated zirconium oxide.
For supporting silver ions on the inorganic compounds, known methods can be employed without limitation. Useful methods include physical or chemical adsorption of silver ions on an inorganic compound; ion exchange reaction for supporting silver ions on an inorganic ion exchanger; bonding of silver ions to an inorganic compound with a binder; embedment of a silver compound into an inorganic compound by impact; vapor deposition; dissolution-precipitation reaction; and thin layer forming processes such as spatter, which form a thin layer of a silver compound on the surface of an inorganic compound.
Among the above methods, ion exchange reaction is preferable since the silver ions can be rigidly supported. Preferred inorganic exchangers include zeolite and zirconium phosphate. Specific examples of antimicrobial agents prepared by this method include commercial products xe2x80x9cNOVALON AG-300xe2x80x9d (silver ion-supporting zirconium phosphate manufactured by Toa Gosei Kagaku K.K.) and xe2x80x9cZeomic AW-10Dxe2x80x9d (silver ion-supporting zeolite manufactured by Shinanen New Ceramic Co., Ltd.).
It is desirable that the silver ion-supporting inorganic antimicrobial agent is in the form of a fine powder with an average particle size of 0.001 to 20 xcexcm, preferably 0.01 to 10 xcexcm, from the viewpoints of finished appearance of the coating film and effective area of the antimicrobial agent.
The proportion of the silver ion-supporting inorganic antimicrobial agent is preferably 0.05 to 50 parts by weight, more preferably about 0.5 to 10 parts by weight, relative to 100 parts by weight of the base resin, from the viewpoints of antimicrobial effect and economy.
Organic antimicrobial agents such as zinc pyrithione include, for example, bis(pyridine-2-thiol-1-oxide) zinc salt. It is desirable that the bis(pyridine-2-thiol-1-oxide) zinc salt is in the form of a fine powder with an average particle size of 0.001 to 20 xcexcm, preferably 0.01 to 10 xcexcm, from the viewpoints of finished appearance of the coating film and effective area of the antimicrobial agent.
The proportion of the bis(pyridine-2-thiol-1-oxide) zinc salt is preferably 0.001 to 20 parts by weight, more preferably 0.05 to 5 parts by weight, relative to 100 parts by weight of the base resin, from the viewpoints of the antimicrobial effect, prevention of discoloration and economy.
The powder coating material (B) may contain, where necessary, an oil repellent, UV stabilizer, UV absorber (such as benzotriazole compound), color pigment, extender pigment, filler, curing catalyst, fluidity modifier, anti-cissing agent or like additive for coating compositions.
In the powder coating composition of the invention, the materials (A) and (B) are used each in a proportion of about 30 to 70 wt. %, in particular about 40 to 60 wt. %.
The powder coating composition of the invention can be prepared by dry blending the two powder coating materials (A) and (B) in a mixer such as a Henschel mixer or a mill such as an atomizer or jet mill.
The powder coating composition of the invention usually has an average particle size of 5 to 100 xcexcm, preferably 10 to 80 xcexcm. If the average particle size is less than 5 xcexcm, the application workability of the powder coating composition lowers, whereas an average particle size exceeding 100 xcexcm lowers the coating efficiency and film appearance.
The powder coating composition of the invention can be applied to a substrate by corona electrostatic coating, frictional electrification coating, fluidized-dipping, hot fluidized-dipping or like powder coating process. The coating film is preferably about 30 to 1000 xcexcm thick, more preferably about 40 to 500 xcexcm thick, when cured. The composition applied is baked usually at about 120 to 200xc2x0 C. for about 10 to 60 minutes.
Usable substrates include conventional substrates amenable to powder coating and free from thermal deformation. Specific examples are those made of iron, steel, copper, stainless steel, alloy steels, aluminum and its alloys, zinc, zinc-plated steels, zinc alloys, tin-plated steels, zinc phosphate- or iron phosphate-treated steels and like metals, and glasses. The substrate may be a plate or a shaped article such as a pipe, box, wire or frame. A primer coating or intermediate coating may be applied to the surface of the substrate.